Summary We have used the previously developed technologies of RNA optimization to optimize expression of IL-15 cytokine, and have shown that we can over-produce bioactive cytokine after DNA delivery in mice and macaques. We explored the biology of IL-15 and showed that efficient production of IL-15 is possible only by co-expression in the same cell with the so-called IL-15 Receptor-alpha. We also showed that a second form of IL-15 (SSP IL-15) previously identified in humans and rodents as intracellular or nuclear IL-15 is also efficiently secreted from the cells when co-expressed with the IL-15 Receptor alpha. These results shed new light in the biology and regulation of IL-15 and provide methods for the efficient production and clinical application of this cytokine. Cell lines overproducing soluble bioactive IL-15/IL-15 Receptor alpha heterodimers have been constructed and were used for the production of the authentic bioactive form of IL-15 found in the body. IL-15 purified from over-producing human cells was injected in mice and shown to be bioactive. IL-15 is of interest due to its ability to stimulate the growth, activation and survival of lymphocytes, including CD8 and NK cells. Thus, IL-15 has been considered for cancer immunotherapy and for support of the growth of cytotoxic cell clones after adoptive transfer. Other proposed uses of IL-15 are in toxic shock and as vaccine adjuvant. We have shown that IL-15 injection accelerates the recovery of lymphocytes in mice rendered lymphopenic after treatment with cytotoxic drugs. We have used optimized expression vectors to express IL-12 cytokine in animals. Efficient expression results in bioactive levels, which increase immune response after DNA vaccination, thus becoming important molecular adjuvant for our vaccines. This work established methods to optimize expression of the IL-12 family of cytokines (IL-12, IL-23, IL-27, IL-35). Efficient expression of IL-27 after DNA delivery demonstrated synergy with IL-2 in the elimination of neuroblastoma metastases in mice. We have previously identified an extensive family of RNA transport elements (RTE) in the mouse genome able to replace the HIV-1 Rev/RRE posttranscriptional regulatory system, using a mutated HIV-1 DNA proviral clone as a novel molecular trap. This is general methodology for the identification of cis-acting posttranscriptional control elements in the mammalian genome. We have identified the cellular factor responsible for binding to RTE and linking it to the NXF1 export pathway. This protein, the RNA binding motif protein 15 (RBM15), had no previous assigned function. Our analysis revealed direct interaction of RBM15 and a related protein, OTT3, with the essential nuclear export factor NXF1 via their C-terminal regions. Biochemical and subcellular localization studies showed that OTT3 and RBM15 also interact with each other in vivo, further supporting a shared function. Genetic knock-down of RBM15 in mouse is embryonic lethal, indicating that OTT3 cannot compensate for the RBM15 loss, which supports the notion that these proteins, in addition to sharing similar activities, have distinct biological roles. RBM15 is an important factor facilitating the function of other proteins on RNA. The DEAD family RNA helicase Dbp5 is essential for nuclear export of mRNA and is thought to dissociate Mex67 from mRNP upon translocation, thereby generating directional passage to the cytoplasm. The molecular mechanism by which Dbp5 recognizes Mex67-containing mRNP is not clear. We found that RBM15 binds specifically to human DBP5 and facilitates its direct contact with mRNA in vivo. These results contribute significantly to the further understanding of the basic mechanisms of nucleocytoplasmic traffic of macromolecules.